The present invention relates to a method and apparatus for determining the rotational speed of a rotary member, and more particularly to a method and apparatus for determining the rotational speed of such a rotary member as an output shaft of an automobile prime mover or an output shaft of a power transmission unit thereby to automatically control in a digital manner the vehicle speed, the operative condition of the prime mover or the like in relation to the determined rotational speed of the rotary member.
In one conventional method of determining the rotational speed of an output shaft of a power transmission unit, a permanent magnet with a plurality of magnetic poles is coupled with a speedometer cable driven by the output shaft, and a reed switch is arranged to magnetically detect the respective magnetic poles of the magnet in sequence so as to produce an electric signal indicative of a time period corresponding to the interval between adjacent magnetic poles of the magnet. In the use of such a permanent magnet, which is commercially available at a low cost, it has been experienced that an irregular interval between adjacent magnetic poles causes an irregularity in the time periods detected by the reed switch, resulting in an erroneous determination of the rotational speed of the output shaft. Due to the erroneous determination of the rotational speed, the proper digital control of the vehicle speed or of the operative condition of the prime mover may not be ensured, and there may occur hunting in the vehicle speed, deviation of the controlled speed from a set speed or the like.
To solve the above problem, it is required to enhance the precision in the position of each of the magnetic poles of the magnet. The requirement, however, may not be economically realized because of the resulting increase of the manufacturing cost of the magnet. In one method of eliminating the error in determination of the rotational speed of the output shaft, it has been proposed to calculate the sum of the time periods detected in sequence by the reed switch during each one rotation of the magnet thereby to regularly detect the rotational time period of the magnet. In the actual practice of the method, however, a smooth, high responsive digital control of the vehicle speed or the operative condition of the prime mover may not be ensured because the regularly detected rotational time period is longer than the individual time periods defined by the intervals between adjacent magnetic poles of the magnet. This is noticeable in the case that a digital computer is adapted to control the vehicle speed or the operative condition of the prime mover.
In another method of determining the rotational speed of an output shaft of an internal combustion engine, a rotary member of magnetic material with a plurality of radial projections is coupled with the output shaft, and an electromagnetic pick-up device is arranged to magnetically detect the respective projections of the rotary member in sequence so as to produce an electric signal indicative of a time period corresponding to the interval between adjacent projections of the rotary member. In the actual practice of the method, unbalance in the air-resistance within an intake manifold causes irregularity in the quantity of air-fuel mixture supplied into respective cylinders of the engine, resulting in irregular rotation of the engine output shaft. This means that the sequence of individual time periods detected by the pick-up device becomes irregular in spite of the regular position of the projections of the rotary member, and that the proper digital control of the operative condition of the engine may not be ensured due to an erroneous determination of the rotational speed of the output shaft caused by the irregularity of the time periods detected. To solve the problem, it has been proposed to calculate the sum of the time periods detected in sequence by the pick-up device during each rotation of the rotary member thereby to regularly detect the rotational time period of the rotary member. In this case, it is also apparent that a smooth, high responsive digital control of the operative condition of the engine may not be ensured for the same reason discussed above.